1. Field of the Invention
This invention relates to an apparatus and method for molding and processing a curable composite patch that can be used in the repair of flat or contoured structures, such as the skin of an aircraft, where the patch must conform to the contour of the surface. The patch, once molded and cured, forms a rigid contoured patch that can be positioned over the damaged area.
2. Description of the Prior Art
Methods of repairing damage to surface structures, such as the body of an aircraft, may be accomplished in a variety of ways. Typically the damage is a hole or tear on the surface structure that can be repaired by patching the damaged area.
One method of patching the damaged area is to process a patch directly on the damaged surface structure. This is done by smoothing out the damaged area so there are no jagged edges or protrusions. A thin sheet of backing material is inserted through the hole or tear with the sheet having a larger surface area than the hole. The sheet is usually flexible so that the sheet can be bent when passed through the hole or tear. A paste adhesive is then applied to the sheet and to the back side of the damaged surface. The thin sheet is supported in place against the back side of the damaged surface while the adhesive cures. The hole is then filled by applying paste adhesive to the sheet so that the paste is level with the outer skin surfaces of the damaged structure. An uncured composite patch is then placed over the damaged area. A flexible vacuum bag or cover is positioned over the patch and damaged area, and sealed around the perimeter. By applying a vacuum to the vacuum bag, the vacuum bag is collapsed and forced against the patch and damaged area. Heat is then applied to the patch by applying a heater blanket so that the patch is processed and bonded to the damaged surface at the same time.
Another method involves the use of a second, undamaged surface structure which is used as a tooling mold for fabricating a contoured composite patch. A release film is first placed over the area where the patch is to be made on the undamaged skin of the second structure. This release film is used so that the uncured composite patch will not adhere to the undamaged structure. The uncured composite patch is positioned on the release film and vacuum bagged so that the patch is forced against the undamaged structure. Heat, supplied from a heater blanket, and vacuum pressure are applied to cure the patch while the patch is maintained on the undamaged structure. When the patch is fully cured, the patch is removed and fastened over the damaged area of the original structure.
The heater blankets used in both methods are usually constructed by sandwiching electric heating elements between layers of fiberglass-reinforced silicone rubber sheets. A controller reads the temperature from a thermocouple attached to the heater blanket and adjusts the electrical current to the heater blanket by comparing the temperature at the thermocouple with a preset temperature. If the actual temperature is less (or more) than the set temperature, the current is increased (or reduced) to the heat blanket.
There are several disadvantages to the methods described above for repairing damaged surface structures with composite patches. The first method is difficult to carry out and may result in the backing material not conforming to the skin curvature due to the complex contours of some structures. There may be even more problems in cases where substructure interferences prevent the thin backing sheet from being easily installed.
The second method has the disadvantage of requiring the use of the second, undamaged structure for the period of time that the patch is being processed. This can typically last from one to four hours. The second method also limits the type of resin that can be used on the composite patch to one that can be processed at a temperature of less than 200 degrees fahrenheit. Temperatures in excess of 200 degrees fahrenheit can result in damage to some materials used in aircraft applications. The second method also becomes difficult when the damage is on the bottom or side surfaces of structures where the weight of the patch and heater blankets tend to move the patch from its proper position. Both methods require that seams and holes on the surface structure be sealed in order to apply a vacuum over the area. This is a difficult task, especially for large areas needing repair.
It is often difficult to obtain even temperature distribution using the heater blankets. The surface structure sometimes acts as a heat sink. This can cause a temperature variation of 20 degrees to 30 degrees F. across the repair patch. Because of the construction of the heater blankets, the blankets will not accommodate complex contoured surfaces.